Chronic diarrhea is a severe clinical disorder in AIDS patients, often contributing substantially to death. The etiology of this syndrome was assessed by extensive cultures and panendoscopy with biopsy. AIDS patients with diarrhea for more than 2 weeks are eligible for study. A complete microbiological evaluation of 3 stools is performed, followed by a malabsorption work-up (fecal fat, C-Sylose absorption, Schilling test) and then upper endoscopy and lower endoscopy with biopsies. Any treatable process identified is treated. The significance of this work is the identification of factors that contribute to the morbidity and mortality of AIDS, particiuylarly the inanition associated with AIDS, so that the currently dreadful prognosis of this underlying disease can be improved. To date, 20 patients and 10 controls have been entered. A likely etiology of diarrhea has been found in over 25% of cases, including Kaposi's sarcoma, cytomegalovirus, giardia, ameba. Therapeutic approaches have been attempted with notable success in two cases of CMV colitis, a previously untreatable process. One patient with a constricting Kaposi's sarcoma lesion was palliated by chemotherapy.